Filter with preselected attenuation/wavelength characteristic

ABSTRACT

A filter has a preselected attenuation/wavelength characteristic, in which spatially separated parts of the filter attenuate different wavelengths. The spatially-separated parts have different attenuation characteristics to attenuate different wavelengths in a predetermined manner to provide a selected attenuation/wavelength characteristic. In one arrangement an interference type filter includes a grating, the pitch of which varies spatially. In one instance, the structure to determine the proportion of radiation subject to interference includes a grating of spatially-varying effectiveness, but alternatively it may include an attenuation filter, the attenuation effect of the attenuation layer varying spatially. In another arrangement, the filter may include structure to separate received radiation into a spatially-disposed spectrum, and to attenuate different parts of the spatially-disposed spectrum in such a manner as to provide the selected attenuation/wavelength characteristic.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a filter with a selectedattenuation/wavelength characteristic, and a method for making the same.

2. Related Art

It is known that the erbium (or other rare earth) doped fibre amplifiersor other types of fibre amplifiers are very attractive devices foroptical communications and are likely to become widely used in place ofoptoelectronic repeaters. However, as it is generally desirable tomultiplex signals by means of wavelength division multiplexing (WDM),the gain spectrum of an erbium doped fibre amplifier (simplified inFIG. 1) is not ideal having one large peak A and a (usually) lower peakB and a flatter spectrum over the desired range R, as shown in FIG. 2,would be desirable, especially where a series of amplifiers are used, toavoid large differences in gain across the wavelength band. Thus if thedifferent signals which are to be multiplexed are provided atwavelengths λ₁, λ₂. . . λ₅ as shown in FIG. 1, the signal at wavelengthλ₂ will be amplified considerably greater than the signals at wavelengthλ₁ and λ₃ or λ₅ and the signal at wavelength λ₄ will be amplified to aslightly less extent than λ₂. This clearly creates problems particularlyif the WDM signal is passed through several erbium doped fibreamplifiers. One method of obtaining a flatter gain spectrum is tointroduce a filter into the system with a selectedattenuation/wavelength characteristic to compensate for the variation ofthe spectrum from a preferred flat spectrum.

It will be understood, in addition to the peaks A and B the erbiumspectrum includes fine detail (not shown). It is possible to eliminatepeak A by means of a filter such as a known interference filter (eg:grating filter) with an attenuation characteristic shown in FIG. 3having a maximum attenuation at C at the interference wavelength and anattenuation peak D at lower wavelengths by matching peak D with peak A.Such an arrangement is described in an article entitled "D-Fibre Filterfor Erbium Gain Spectrum Flattening" (Electronics Letters, 16 Jan. 1992Vol 28, No 2, page 131-132). Such a filter can improve the flatness ofthe gain spectrum to within 0.5 dB over a 30 nm span. FIG. 4 shows theerbium gain spectrum after passing through the filter with thecharacteristic of FIG. 3. Nevertheless, peak B of the amplifier gainspectrum remains, and indeed this feature can be accentuated when thesignal is passed through a succession of erbium doped fibre amplifierswith such filters.

It will be understood that a filter for use with an erbium or other rareearth doped fibre amplifier should not be a reflection type filter sincethis will create problems with the amplifier. A preferred type of filteris an interference filter, and we will describe a variety of filtersincluding Bragg side-tap gratings.

In this specification the term "optical" is intended to refer to thatpart of the electromagnetic spectrum which is generally known as thevisible region together with those parts of the infra-red andultraviolet regions at each end of the visible region which are capable,for example, of being transmitted by dielectric optical waveguides suchas optical fibres.

SUMMARY OF THE INVENTION

The present invention provides, according to a first aspect, a filterwith a selected attenuation/wavelength characteristic over a range ofwavelengths in which the filter extends in a generally linear manner,the filter being adapted so that the wavelength of radiation which thefilter attenuates varies continuously from a first part of the filterwhich attenuates a first wavelength to a second part, spaced from thefirst part, which attenuates a second wavelength, the first and secondwavelengths defining the range of wavelengths, the filter being arrangedso that the degree of attenuation varies in a selected manner from saidfirst part to said second part to provide said attenuation/wavelengthcharacteristic.

In the arrangement to be described, the filter may be provided with apreselected attenuation/wavelength characteristic to match the spectrumof an amplifier such as an erbium doped fibre amplifier but thepreselected attenuation/wavelength characteristic may be chosenaccording to the circumstances of the filters. For example, a filterwith a different preselected attenuation/wavelength characteristic maybe provided to attenuate a different spectrum or indeed may be arrangedto provide a particular desired spectrum in response to a flatter inputwavelength spectrum.

The filter is preferably an interference type filter, the dimensions ofinterference means of said interference filter providing theinterference and defining the wavelength varying in a continuous manneracross the filter between said two parts, and attenuation means isspaced in a continuous manner across said filter to determine theproportion of radiation subject to interference at each point acrossfilter.

In one arrangement said interference type filter includes a grating, thepitch of which varies spatially.

In one instance, the attenuation means comprises a grating ofspatially-varying effectiveness, but alternatively the attenuation meansmay comprise an attenuation filter, the attenuation effect of theattenuation layer varying spatially.

In another arrangement, said filter may include means to separatereceived radiation into a spatially-disposed spectrum, and means isprovided to attenuate different parts of the spatially-disposed spectrumin such a manner as to provide said selected attenuation/wavelengthcharacteristic.

The present invention also provides a method for producing a filtercomprising passing to a radiation transmitting material with defectstates two beams of radiation of a wavelength which change the densityof defect states, said radiation being provided in the form of two beamswhich provide an interference pattern on said material, one of the beamsbeing divergent or convergent with respect to the other beam whereby toprovide an interference grating pattern of density of defect states withspatially varying pitch, and further modifying the density pattern ofdefect states spatially across the interference pattern either as theinterference pattern is produced or thereafter to provide a filter asaforesaid.

The present invention also provides a filter with a preselectedattenuation/wavelength characteristic, in which spatially separatedparts of the filter attenuate different wavelengths, saidspatially-separated parts having different attenuation characteristicswhereby to attenuate different wavelengths in a predetermined manner toprovide a selected attenuation/wavelength characteristic.

Preferably the filter comprises a plurality of side-tap Bragg gratingsin a length of optical fibre. The advantage of such an arrangement isthat the radiation is not reflected back along the optical fibre and thefilter may be used with a erbium or other rare-earth doped fibreamplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described by way ofexample only with reference to the accompanying drawings in which:

FIG. 1, already described, shows, in diagrammatic form, the gainspectrum of an erbium doped fiber amplifier,

FIG. 2, already described, shows, in diagrammatic form, the desired gainspectrum of an erbium doped fibre amplifier with filter,

FIG. 3, already described, shows, in diagrammatic form, anattenuation/wavelength characteristic of a known interference filter,FIG. 4, already described, shows, in diagrammatic form, the gainspectrum of an erbium doped fibre amplifier after passing through afilter with characteristics shown in FIG. 3,

FIG. 5, shows, in diagrammatic form, a desired attenuation/wavelengthcharacteristic of a filter according to the invention,

FIG. 6 sets out diagrammatically the arrangement of amplifier and filterfor use in an optical fibre,

FIG. 7 shows an axial section of "D" fibre illustrating diagrammaticallya first method for producing a grating type filter of the invention,

FIG. 8 is a diagrammatic graph of the density of GeO₂ defects along theaxis of the grating formed by the method of FIG. 7,

FIG. 9 shows a second method for manufacturing a grating filter of theinvention,

FIG. 10 is a diagrammatic view of a second type of filter of theinvention,

FIG. 11 is a cross section on line 11-11 of the filter of FIG. 10,

FIG. 12 shows, for a third type of filter of the invention, the measuredtransmission loss spectrum,

FIG. 13 shows the gain spectrum of the amplifier used in saturationbefore passage through the third type of filter of the invention,

FIG. 14 shows the measured loss spectrum of individual parts of thefilter as well as the composite response of all of the filters, and,

FIG. 15 is the flattened gain spectrum of the amplifier after passingthrough the third type of filter.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring to FIG. 6, there is shown a mono mode optical fibre 10 forreceiving input signals at input 10A and passing them to output 10B. Asdescribed above, the input signal passed to the input 10A may comprise awavelength division multiplexed signal, each separate signal carrying,for example, a separate telephone channel. An erbium doped fibreamplifier 14 is provided comprising a semiconductor laser 11 whichprovides pumping radiation of for example 0.98 micron or 1.48 micronalong a fibre 12 to an energy transfer section 13 of the fibre 10. Thereis also provided a filter 16 at the output of the amplifier 14 whichprovides a selected attenuation/wavelength characteristic.

As has already been described with reference to FIG. 1, the erbium dopedfibre amplifier 14 is arranged to amplify input data signals provided atthe input 10A of the fibre 10, the arrangement being such thatmultiplexed input data signals are provided at the input 10A, in awavelength range R which can be amplified by the amplifier 14. The gainspectrum of the amplifier is shown in FIG. 1 and it will be seen thatthose multiplex signals of a wavelength λ₂ or λ₄ adjacent the large peakA or the lower peak B will be preferentially amplified and it istherefore desirable to provide a filter which, in combination with thefibre amplifier 14, provides an overall amplification which issubstantially flat across the waveband being used as shown in FIG. 2.

The arrangement of the erbium doped fibre amplifier 14 is well known andwill not be described further.

The preferred embodiments of the invention will relate in this case tothe filter 16 which is provided with selected attenuation/wavelengthcharacteristics which compensate for the amplifier gain spectrum.

Fibre grating devices for use as reflection filters have previously beenproposed. A grating is provided on the inner core of the optical fibreby utilising a D fibre section and providing, for example, transverseslots which act as an interference grating. Such a grating has apredetermined pitch distance between the slots which is chosen so as toprovide an interference attenuation characteristic of FIG. 3 whicheliminates, for example, the peak A in the erbium doped fibre amplifiergain characteristic as shown in FIG. 4.

In the present arrangements of the invention to be described withreference to FIGS. 6-9, there is provided a grating in which the pitchvaries along the grating so as to provide a grating which will provideinterference with radiation of a range of values of wavelength. Whilst aslot type grating of the type described in the preceding paragraph maybe provided with a pitch which varies along the grating, in practice weprefer to provide an alternative form of grating which can be moresimply manufactured.

FIG. 7 shows an arrangement for producing a grating of variable pitch.The fibre 10 which, as is well known comprises an core 10D of, forexample, silicon dioxide (SiO₂) doped with germanium dioxide (GeO₂)surrounded by an outer cladding 10E of SiO₂, and in the area of thegrating, the fibre is cut or ground away to a semicircular section toprovide a "D" cross section fibre. Within the core 10D, the germaniumdioxide includes defect states. These defect states affect the opticalcharacteristics of the core. Illumination of the core 10D by means ofradiation of a wavelength of about 240 nm changes the state of thesedefects and thereby changes the effective refractive index of the innercore. Referring to FIG. 7 it will be seen that the core 10D of the fibre10 is illuminated by two beams of radiation 20A, 20B of 240 nmwavelength, the two beams 20A, 20B being passed to the core 10D onrespective optical axes 21A, 21B at an angle to one another whereby aninterference pattern 22 is formed on the core 10D. It will also be notedthat one of the beams 20A, is arranged to be divergent and as a resultan interference pattern 22 of variable pitch is provided. A typicalpitch for a 1.5 μm reflecting grating would be, say, 0.4 to 0.7 μm, say0.53 μm. With a 240 nm beam this corresponds to a 14 degree half-anglebetween the interfering beams, and a typical divergence angle would be 1degree. Note that both beams could be diverging, rather than just one.The variation in pitch required scales directly with the spread requiredin the wavelength, 5% pitch variation for 5% wavelength spread.

It will be understood that the interference pattern 22 of radiation (of240 nm) produced, which is of grating form, will produce a correspondinggrating type pattern in the density of the germanium oxide defectstates. FIG. 8 illustrates in exaggerated form the density of defectsalong the axis of the grating 22. As thus far described, with the twobeams 20A and 20B being passed to the core 10D, the density of defectswill be sinusoidal but with increasing pitch, as illustrated by thesolid line in FIG. 8, sections a,e,b,f,c,g,d. In this way there isprovided, effectively, on optical grating which will provide aninterference optical effect for the radiation passing along the core 10Dwhich exactly mirrors the interference pattern 22 produced by the beams20A, 20B.

By selecting the relative angles between the two beams 20A, 20B, we canprovide an interference pattern 22 of a desired basic pitch and byselecting the angle of divergence of the beam 20A, we can selectivelyvary the pitch along the grating.

As a result, there is provided an interference filter which providesinterference with a selected range of wavelength values of radiationpassing along the core 10D, and we select the range of wavelength valuesto match the range of WDM signals to be used, ie, a range of at least λ₁-λ₅ this range being indicated as R in FIG. 1.

It is also desired to provide a variable degree of interference effectat different wavelengths across the range R of wavelengths and this canbe provided during manufacture in two ways. In the first arrangement,there may be provided as shown in FIG. 7 an attenuating layer 23 whichmay be of any material which attenuates the 240 nm radiation in thebeams 20A, 20B, the thickness of the attenuating layer 23 being arrangedspatially with respect to the grating so as to provide a desiredattenuation effect along the length where the grating is to be formed,to thereby vary the effectiveness of the interference of the grating asdesired. In an alternative arrangement, the layer 23 instead of being ofvariable thickness, may be of variable density (which may be providedphotographically, for example).

The net effect of the attenuating layer 23 (assuming for the moment thatit is an equal thickness along the ridge illustrated in FIG. 7) is toreduce the effect of the radiation in reducing the number of defectstates and so the density of defects will follow parts a,m,b,n,c,p,d inthe graph of FIG. 8. In other words, the depletion of the density of thedefect states will be reduced. In practice, as the thickness of thelayer 23 varies across the grating, the minimum value of density ofdefects will vary towards zero where the layer 23 is thinnest to a valueclose to the initial value D_(o) where the layer 23 is thickest.

In this way, there is provided a grating 26 in the core 10D which may bearranged to interfere with a range R of wavelengths of radiation passingalong the core 10D, different parts of the grating interfering withdifferent wavelengths, and the interference effect may be varied atdifferent wavelengths by selectively varying the extent of interferenceeffect at different spatial parts of the grating to provide a desiredattenuation/wavelength characteristic. In a preferred arrangement, thecharacteristic provided may be as shown in FIG. 5. In this way, theoutput of an erbium doped fibre amplifier can be provided which issubstantially flat across a range of wavelength values as shown in FIG.2. It should be understood that the filter may be arranged not only toeliminate the peaks A and B of the amplifier gain spectrum but also thefine detail not shown in FIGS. 1-5.

In an alternative, the variable pitch grating may be provided as shownin FIG. 7 but without the attenuating layer 23. In order to provide thevariable interference effect across the grating and hence across thewavelength range, a beam of 240 nm radiation may be swept across thegrating 26 after it has been formed and the intensity of the beam variedso as to modulate the effectiveness of the grating in different parts ofthe grating. In this case, viewing FIG. 8, the effect will be thatinitially before the last beam of radiation is swept across the grating26, the density of defects will follow the lines a,e,b,f,c,d,g,d butafter the radiation has been swept across the grating 26, the density ofdefects will follow the line h,e,j,f,k,g,l. The shape of the graph willbe the same as for the previous arrangement but with the relevant valueshifted downwards in FIG. 8. Such an arrangement is shown in FIG. 9 inwhich there is provided a beam 27 of such radiation, and a moveable slit28 is passed over the grating, the intensity of the beam being varied inaccordance with a prearranged pattern as the slit 28 passes over thegrating 26.

The above methods described with reference to FIGS. 7 and 8 show a"D-fibre". In practice, the same methods may be used with a simplecircular fibre with core or indeed with a rectangular sectionsemiconductor waveguide.

FIGS. 10 and 11 show an alternative embodiment of filter according tothe invention.

A second embodiment of filter with a selected attenuation/wavelengthcharacteristic will no be described with reference to FIGS. 10 and 11.Referring to FIG. 10 there is shown a planar waveguide 40 comprising aplurality of layers of material. As shown in FIG. 11, there is a baselayer 41 of silicon, a layer 42 of silicon dioxide, a layer 43 ofsilicon dioxide and germanium dioxide GeO₂, and an upper layer 44 ofsilicon dioxide. In a sense, the layers 42 and 44 correspond to theouter cladding 10E of the arrangement of the earlier drawings, and thelayer 43 corresponds to the core 10D.

The waveguide 40 is generally rectangular and there is provided an inputport 46 and an output port 47 in the edge 48 of the waveguide 40.Opposite the input port 46 there is provided a first curved mirrorgrating 51, the mirror comprising a series of stepped mirrors 51A,B . .. as shown in exaggerated form in the circle in FIG. 10. In practice themirror grating 51 is formed by etching the layer 43 in the region 53. Asimilar second curved mirror grating 52 is provided opposite the outputport 47. A linear attenuation means 54 is provided between the gratings51, 52. The form of this attenuation means 54 will be described later.

Referring to FIG. 10 it will be seen that the fibre 10 carrying awavelength division multiplexed signal as before may be connected to theinput port 45 and radiation is thereby transmitted through the layer 43to the first mirror grating 51. The radiation is reflected from thatfirst mirror grating 51 to the second mirror grating 52 and thence theoutput port 47. The mirror grating 51, however, causes interference andthereby provides a spatially-separated spectrum of the wavelengths ofthe incident radiation from λ₁ to λ₅ as illustrated along the length ofthe linear attenuation means 54. The mirror grating 52 recombines thespectrum into a single WDM output signal.

Effectively, therefore, the range R of wavelengths of interest areseparated along the length of the linear attenuation means 54 and aswill be apparent, by varying the attenuation effect of the linearattenuation means 54 along its length, we can provide a selectedattenuation/wavelength characteristic.

Various linear attenuation means 54 may be provided. In a firstarrangement, a slot may be cut or etched along the line X-Y to below thelayer 43 and an attenuation filter inserted in the slot. The filter maybe produced photographically, and the density of the filter may bevaried along the line X-Y in accordance with a predeterminedcharacteristic to provide the necessary selected attenuation/wavelengthcharacteristic.

Alternatively, there may be provided over the line X-Y a layer of highrefractive index material 56 (see FIG. 11) the width of which parallelto the surface of base layer 41 and orthogonal to X-Y may be varied soas to provide a variable effect on the transmission (and henceattenuation) of radiation across the line X-Y.

By varying the dimensions of the mirror grating 51 we select the range Rof wavelengths which are to be separated along the length of the linearattenuation means 54, and by varying the attenuation characteristic ofthe linear attenuation means 54 in accordance with the wavelength of thespectrum passing that particular part of the attenuation means, we canvary the attenuation/wavelength characteristic so that, as with thepreceding example, we can provide a filter having awavelength/attenuation characteristic as shown in FIG. 5 which not onlycan eliminate or reduce the peaks A and B in the gain characteristic ofthe amplifier, but can also eliminate fine detail not shown.

FIGS. 6,7,10 and 11 show diagrammatically the apparatus of theinvention. In practice there will also be provided isolators to rejectradiation reflected back along the incident path or alternatively thegrating(s) could be tilted off-axis to ensure that reflected beams donot couple back into the fibre.

We will now describe a third filter of the invention. In principle, thethird type of grating comprises a plurality of photosensitive side-tapgratings written directly into optical fibre by selection of the desirednumber of phase masks, each with a different pitch or period, to give a5 nm wavelength separation in the written gratings. Thus along thelength of the fibre, there are provided a succession of Bragg side-tapgratings. The gratings are written using intra-cavity frequency doublingin a BBO crystal of an argon laser, providing over 100 mW of 224 nmwavelength radiation. The central wavelengths of each side tap gratingare defined by specially designed e-beam written transmission phasegratings together with a simple interferometer to recombine the beams.The diffracted beams pass through a rectilinear prism and are totallyinternally reflected to recombine at the fibre. Several phase gratingscan be fabricated on a three inch ultra violet wavelength-transmittingsilica substrate, with each grating period. defining a wavelength spacedby 5 nm from its near neighbours to thereby provide a set of gratingperiods which are spaced apart by 5 mm over a band of wavelengths from1530 to 1570 nm. FIG. 12 shows the measured transmission loss spectrumof eight photosensitive side-tap gratings written directly on to theoptical fibre by selection of eight phase masks each with a differentperiod to give a 5 nm wavelength separation in the written gratings. Thephotosensitive gratings are then written in the manner described in RKashyap, R Wyatt, R J Campbell, "Wideband gain flattened fibre amplifierusing a photosensitive fibre blazed grating", Electronics Letters 29(2),154-155, 21 Jan. 1993 and R Kashyap, J R Armitage, R J Campbell, D LWilliams, G D Maxwell, B J Ainslie and C A Millar, "Light sensitiveoptical fibres and planar waveguides", BT Technology Journal 11(2)150-160, 1993 (both of which are herein incorporated by reference) butwith a slightly shallow angle of 6°. FIG. 12 shows the loss spectrum foreight gratings written into highly photosensitive fibre as before. Thegratings were written in sequence and the peak loss for each grating wasmonitored in real time using an erbium fibre ASE source. The peak lossat 1548 nm of 4 dB was the result of writing two such gratings. Fromthis Figure one can see that side-tap loss may be induced at will at thedesired wavelengths. The widths and the central wavelength can becontinuously adjusted by alignment with a fibre.

Firstly, the saturated gain of the 1480 nm diode pumped amplifier wasmeasured under normal operating conditions. This is shown in FIG. 13which shows the gain spectrum of the amplifier used in saturation beforeflattening. By suitably selecting gratings from the data set of FIG. 12,a design of the filter was established, providing the information on thepeak loss and position of each grating. The required gratings werewritten into a 15 cm length of highly photosensitive fibre (as by themethod described in D L Williams, B J Ainslie, J R Armitage, R Kashyap,R J Campbell, "Enhanced photosensitivity of boron doped optical fibres",Electronics Letters 229(1), 45, 1993, incorporated herein by reference)by preselecting the appropriate phase-mask. The cumulative loss of thegratings is shown immediately above individual filters (bottom trace) inFIG. 14. FIG. 14 shows the measured loss spectrum of individual fibres(bottom trace) written into the optical fibre following the requirementsfor the desired filters. (i), (ii) and (iii) are filters written afterthe amplifier was connected. The upper most trace shows the compositeresponse of all the fibres when the required loss for all the gratingshad been achieved using the amplifier in saturation. The flattened gainspectrum with the filter placed after the amplifier, is shown in FIG. 15(bottom trace). FIG. 15 shows the flattened gain spectrum of theamplifier with the filter in-line (upper trace). The final spectrum(lower trace) was achieved after three further gratings (i), (ii) and(iii) were written. The two small peaks around that were written bywriting three more gratings each with a peak loss of approximately 0.5dB at 1544, 1531 and 1532 nm [also shown in FIG. 14 as (i), (ii) and(iii)]. The final spectrum is shown in FIG. 15 (top trace). This shows aflattened gain spectrum to within ±0.3 dB from 1532.5 nm to 1565 nm.

The overall filter gain when compared with the original amplifierspectrum shows a difference of 2.2 dB at the minimum gain point at 1537nm. This difference would ideally be zero. 1.9 dB of loss is attributedto the two splices to the FC-PC connectors, owing to the mismatchbetween the fibres used for the filter and the Type "B" fibres in theconnectors. The insertion loss incurred by the writing of the filter istherefore approximately 0.3 dB. This is due to the loss spectrum of thegratings and is a compromise between the degree of flattening of thespectrum and the loss incurred. There is no measurable loss due tobroadband coupling to radiation modes. The output power in the saturatedregime for the filtered amplifier was +7 dBm of the design input of -5dBm. This loss of output power is due to the simple configuration of theexperiment; an optimised amplifier would have the filter placed withinthe amplifying fibre. With careful splicing, the insertion loss shouldbe little greater than the insertion loss of the filter.

It has been shown that photosensitive side-tap Bragg gratings may beused extremely effectively to equalise the gain spectrum of erbium dopedpower amplifiers to within ±0.3 dB over 33 nm with a 0.3 dB losspenalty. This scheme is also highly attractive in tailoring thetransmission spectrum of a wide variety of devices compatible withoptical fibre. Different shaped filters may be made by suitable choiceof the peak loss wavelength of each grating. It has also been shown thatit is easy to tap light out of the fibre at any desired wavelength usingthis non-invasive technique. The side-tapped power may be used formonitoring purposes or for non-invasive signal reception in a ringcommunication system.

It will be understood therefore, that by providing more than one gratingat a particular chosen frequency, one can provide a filter which has adesired characteristic. In the arrangement shown in FIG. 12, the filteris clearly more effective at a wavelength of 1548 than at otherwavelengths.

To design a suitable fibre to flatten the gain spectrum of an amplifier,the saturated gain of a 1480 nm diode pumped amplifier was measuredunder normal operating conditions. The spectrum is shown in FIG. 13. Bysuitably selecting gratings from the data set shown in FIG. 12, a designof filter can be established. The required gratings are written into a15 cm length of highly photosensitive fibre by preselecting theappropriate phase masks. The cumulative loss of the gratings is shown.

In summary of the third embodiment of the invention and the arrangementsof FIGS. 12 to 15, we may provide in an optical fibre a plurality ofshort lengths of side-tap Bragg gratings, each of the short lengthshaving an individual pitch distance which attenuates, along the fibre,radiation of a particular wavelength, the individual gratings havingdepletion densities either all the same density, or individually chosendensities to provide different degrees of attenuation, and for moreattenuation of particular wavelengths, an individual grating may be ofextended length or there may be provided more than one grating of aparticular pitch.

In the arrangement shown in FIG. 12, there is shown the loss/wavelengthcharacteristic of eight gratings, arranged to attenuate wavelengthsspaced by 5 nm, and each having substantially the same depletion densityso as to attenuate the radiation to the same extent, except that thegrating with is arranged to attenuate at 1548 nm is either of twice thelength of the other gratings, or two separate gratings are provided sothat the loss at that wavelength is approximately double that at otherwavelengths.

It will be clear from FIG. 12 that there is overlap in the attenuationcharacteristic of the separate gratings.

FIG. 13 shows the characteristic of an erbium doped amplifier which isto be smoothed and this done by using a number of gratings as set out inFIG. 14. Initially, five gratings are formed and their characteristicsare illustrated as follows:

grating 1 (small solid squares) 2.7 dB loss centred 1530 nm,

grating 2 (small open squares) 1.8 dB loss centred on 1549 nm,

grating 3 (small solid diamonds) 1 dB loss centred 1553 nm,

grating 4 (small open diamonds) 2.5 dB loss centred 1558 nm, and

grating 5 (small solid triangles) 1 dB loss centred 1563 nm.

These five gratings provide a combined attenuation characteristic of thegratings (1) to (5) indicated by the trace indicated by the large solidsquares.

Referring to FIG. 15, this last trace of large solid squares from FIG.14 (the combined attenuation characteristic of gratings 1-5) is shown asthe lower most trace (now illustrated by open squares) and the desiredcharacteristic is shown by the higher trace, that is the trace with thesolid square symbols. It is therefore necessary to "tweak" thecharacteristic. This can be done by providing three further gratings.Because we do not wish to attenuate the radiation too much, it isdesired not to attenuate any further at the lower most point of thetrace at 1538 nm and so we select three further gratings, as follows:

grating (i) (small solid circles) 0.5 dB centred at 1544 nm,

grating (ii) (small open circles) 0.5 dB centred at 1531 nm, and

grating (iii) (small open triangles) 1 dB centred at 1532 nm.

Therefore in a practical environment one may readily produce a filterwhich might be, for example, comprised of the gratings 1 to 5, and onecan then fine tune the filter by providing variable, for each case,gratings (i), (ii), (iii).

We have found in practice that we can provide in a short length ofoptical fibre a side-tap Bragg grating which can reduce a gain variationof one plus or minus 1.6 dB over a bandwidth of 33 nm in a saturatederbium doped fibre amplifier to plus or minus 0.3 dB. The side-tapfilters are of course non-reflective as is essential with the use of anerbium doped fibre amplifier.

The invention is not restricted to the details of the foregoingexamples.

We claim:
 1. An optical fibre amplifier having a selected gain spectrumcomprising:an optical amplifier having a gain spectrum over a range ofwavelengths and a filter with a selected attenuation versus wavelengthcharacteristic over said range of wavelengths in which the filterextends in a generally linear manner, the filter having a non-uniformphysical structure therealong so that the wavelength of radiation whichthe filter attenuates varies continuously and non-uniformly from a firstpart of the filter which attenuates a first wavelength to a second part,spaced from the first part, which attenuates a second wavelength, thefirst and second wavelengths defining the range of wavelengths, thefilter being arranged so that the degree of attenuation varies in aselected non-uniform manner from said first part to said second part toprovide said attenuation versus wavelength characteristic, saidattenuation versus wavelength characteristic being selected with respectto said gain spectrum of said optical amplifier to provide said selectedgain spectrum of said optical fibre amplifier.
 2. An optical fibreamplifier as in claim 1 wherein the filter is an interference filter,the dimensions of interference structure of said interference filterproviding the interference and defining the wavelength varying in acontinuous manner across the filter between said two parts,andattenuation means being spaced in a continuous manner across saidfilter to determine the proportion of radiation subject to interferenceat each point across filter.
 3. An optical fibre amplifier as in claim 2wherein said interference filter includes a grating, the pitch of whichvaries spatially.
 4. An optical fibre amplifier as in claim 3 whereinthe attenuation means to determine the proportion of the radiationsubject to interference comprises a grating of spatially varyingeffectiveness.
 5. An optical fibre amplifier as in claim 3 wherein thepitch of said grating is 0.4 to 0.7 μm, the spatial variation from saidfirst part to the second part being 5%.
 6. An optical fibre amplifier asin claim 3 wherein the pitch of said grating is substantially 0.53 μm,the spatial variation from said first part to the second part being 5%.7. An optical fibre amplifier as in claim 2 wherein the attenuationmeans comprises an attenuation layer, the attenuation effect of theattenuation layer varying continuously spatially across the filter. 8.An optical fibre amplifier as in claim 2 wherein said filterincludes:means to separate received radiation into a spatially disposedspectrum, and means to attenuate different parts of the spatiallydisposed spectrum in such a manner as to provide said selectedattenuation versus wavelength characteristic.
 9. An optical fibreamplifier as in claim 1 in which said optical amplifier comprises anerbium doped fibre amplifier.
 10. A method for producing an opticalfibre amplifier including a spectral filter having a selected gainspectrum over a range of wavelengths, said method comprising the stepsof producing said filter by:passing to a radiation transmitting materialwith defect states two beams of radiation which change the density ofdefect states, said radiation being provided in the form of two beamswhich provide an interference pattern on said material, one of the beamsbeing divergent or convergent with respect to the other beam to providean interference grating pattern of density of defect states withspatially varying pitch, and further modifying the density pattern ofdefect states spatially across the interference pattern either as theinterference pattern is produced or thereafter to provide a filterhaving an attenuation versus wavelength characteristic selected withrespect to a gain spectrum of said optical amplifier without the filterto provide said selected gain spectrum for said optical fibre amplifierwith said filter.
 11. A method as in claim 10 in which the densitypattern of defect states is modified as the interference pattern isprovided by passing said beams to the material through a filteringmedium of spatially varying attenuation property.
 12. A method as inclaim 10 wherein the pattern density of defect states is modified afterthe interference pattern is produced by passing a beam of said radiationof varying intensity across the interference pattern.
 13. An opticalfibre amplifier comprising:an optical amplifier having a gain spectrumover a range of wavelengths and filter with a selected attenuationversus wavelength characteristic, said filter comprising a plurality ofdifferent filter parts, each adapted to attenuate respective differentwavelengths, each said different filter part having a respectiveattenuation characteristic to attenuate its respective wavelength to apredetermined extent, and means to combine the attenuated signalwavelengths from said different filter parts to provide a selectedattenuation versus wavelength characteristic, said attenuation versuswavelength characteristic being chosen with respect to said gainspectrum of said optical amplifier to provide a selected gain spectrumfor said optical fibre amplifier.
 14. An optical fibre amplifier as inclaim 13 wherein said wavelengths are of optical wavelength.
 15. Anoptical fibre amplifier as in claim 13 wherein said different filterparts are spatially separated.
 16. An optical fibre amplifier as inclaim 13 wherein said different filter parts comprise a plurality ofside-tap Bragg gratings in a length of optical fibre.
 17. An opticalfibre amplifier as in claim 13 wherein said filter is an interferencefilter, and in which said different filter parts comprise:separategrating means, the pitch of each said grating means being predefined,and at least some of said grating means having pitches differing fromone another.
 18. An optical fibre amplifier as in claim 17 wherein someof said grating means have the same pitch to increase the attenuation ofradiation of that respective wavelengths.
 19. An optical fibre amplifieras in claim 13 wherein the filter is an interference filter, said filterparts comprising:an interference filter means in which the dimensions ofa means which defines the wavelength which is interfered varies in acontinuous manner across the interference filter means to provide saidfilter parts, and attenuation means provided in a continuous manneracross said filter to determine the proportion of radiation of therelevant wavelength subject to interference at each point across theinterference filter means.
 20. An optical fibre amplifier as in claim 19wherein said interference filter means includes a grating, the pitch ofwhich varies spatially.
 21. An optical fibre amplifier as in claim 20wherein the attenuation means comprises a grating of spatially varyingeffectiveness.
 22. An optical fibre amplifier as in claim 20 wherein thepitch of said grating is 0.4 to 0.7 μm, the spatial variation across thegrating being approximately 5%.
 23. An optical fibre amplifier as inclaim 20 wherein the pitch of said grating is approximately 0.53 μm, thespatial variation across the grating being approximately 5%.
 24. Anoptical fibre amplifier as in claim 19 wherein the attenuation meanscomprises an attenuation layer, the attenuation effect of theattenuation layer varying continuously spatially across the filter. 25.An optical fibre amplifier as in claim 13 wherein said different filterparts includes:means to separate an input signal into a spatiallydisposed spectrum, and means to attenuate different parts of thespatially disposed spectrum so as to provide said amplitude versuswavelength characteristic.
 26. An optical fibre amplifier as in claim 13wherein said optical amplifier comprises an erbium doped fibreamplifier.
 27. An optical fibre amplifier made by the process of claim10.
 28. A method for producing an optical filter having a selectedattenuation spectrum, said method comprising the steps of:passing to aradiation transmitting material with defect states two beams ofradiation which change the density of defect states, said radiationbeing provided in the form of two beams which provide an interferencepattern on said material, one of the beams being divergent or convergentwith respect to the other beam whereby to provide an interferencegrating pattern of density of defect states with spatially varyingpitch, and further modifying the density pattern of defect statesspatially across the interference pattern either as the interferencepattern is produced or thereafter to provide a filter having anattenuation versus wavelength characteristic providing said selectedattenuation spectrum.
 29. An optical fibre amplifier having a selectedgain spectrum comprising:an optical amplifier having a gain spectrumover a range of wavelengths and a filter made by the process of claim 28with a selected attenuation versus wavelength characteristic over saidrange of wavelengths in which the filter extends in a generally linearmanner, the filter being adapted so that the wavelength of radiationwhich the filter attenuates varies continuously from a first part of thefilter which attenuates a first wavelength to a second part, spaced fromthe first part, which attenuates a second wavelength, the first andsecond wavelengths defining the range of wavelengths, the filter beingarranged so that the degree of attenuation varies in a selected mannerfrom said first part to said second part to provide said attenuationversus wavelength characteristic, said attenuation versus wavelengthcharacteristic being selected with respect to said gain spectrum of saidoptical amplifier to provide said selected gain spectrum of said opticalfibre amplifier.
 30. A method of amplifying optical signals over a rangeR of wavelengths λ₁ -λ_(n) with a flattened signal gain over thespectral range R of amplified signals, said method comprising the stepsof:passing said optical signals through an optical fibre amplifierhaving plural peak amplification gain factors over said range R; andsimultaneously also passing said signals through an optical attenuatorhaving plural peak attenuation factors over said range R, said peakattenuation factors being approximately matched to said peakamplification factors over said range R to produce a composite amplifieroutput having a flattened signal gain over the spectral range R ofamplified signals.
 31. An optical signal amplifier for amplifyingoptical signals over a range R of wavelengths λ₁ -λ_(n) with a flattenedsignal gain over the spectral range R of amplified signals, saidamplifier comprising:an optical fibre amplifier having plural peakamplification factors over said range R; and an optical attenuatordisposed within or in series with said optical fibre amplifier, saidattenuator having plural peak attenuation factors over said range R,said peak attenuation factors being approximately matched to said peakamplification factors over said range R to produce a composite amplifieroutput having a flattened signal gain over the spectral range R ofamplified signals.